In shearography, a surface being observed is illuminated by an expanding laser beam, and two time sequential images are captured of the surface of a part with an image-shearing camera. The first image is taken of the surface, and the second image is taken of the same surface a short time thereafter after some deformation of the surface. The two images taken are processed together to produce a third image showing a fringe pattern that depicts the gradient of the displacement of the surface due to some loading of the surface between the first and second images.
More particularly, shearography is an optical measuring technique using coherent light, for the interferometric observation of the surfaces of parts typically under non-destructive thermal or mechanical loading to distinguish between structural information and anomalies of the surfaces of parts due to loading such as thermal or mechanical loading. The two images are each laterally displaced images taken of the surface of the part being observed and the two images are coherently superposed. The lateral displacement is called the shear of the images. The superposition of the two images is called a shearogram, which is an interferogram of an object wave with the sheared surface wave as a reference wave.
The absolute difference of two shearograms recorded at different physical loading conditions of the sample part is an interference fringe pattern which is directly correlated to the difference in the deformation state of the sample part between taking the two images thereof. In contrast to holographic interferometry the fringe pattern indicates the slope of deformation rather than the deformation itself. Defects inside the sample part will affect the local surface deformation induced by the loading and result in a disturbance of the loading fringes that is detected.
The resultant difference images always exhibit a very noisy structure. This is due to what are called speckles. Speckles are statistical interference patterns which occur after reflection of a coherent wave off a rough surface giving the image a grainy structure. Regarding shearography the speckles are the carrier of information, coding the wave field and surface state information respectively and giving rise to interference fringe patterns. However the grainy nature of the speckles is conserved and significantly decreases contrast and signal to noise ratio of the difference images.
The difference images typically exhibit strong noise and low contrast that require further image processing. This further image processing can be either image improvement or image evaluation. The goal is to remove speckle noise and to increase fringe contrast in order to improve the visibility of the fringes.
One shortcoming of existing shearography techniques is that they do not accommodate collecting shearography data from moving platforms such as aircraft, surface craft, or handheld devices. Thus, there is a need in the art for a way to collect shearography data from a moving platform.